Power and authority are not the same thing at all. Totalitarian governments might have power, but that doesn't give them any moral authority. Indeed, I doubt authority ever has its source in some powerful person: we'd need some explanation for why that powerful person counts as having authority, and it would be circular to say they authoritatively bestowed authority on themselves.

This is just semantics though because when I used the words power and authority, I just used them to mean the same thing. If we want to further define what these words mean, feel free, but when I used 'power', I simply mean a higher authority.

We're considering a case where the atheist doesn't accept moral objectivity, so it would make no sense for the theist to say that.

Yes, the atheist will use the objective morals based in theism to prove the theists wrong, yet the theists believe they get their objective morals from God...so the atheist will have to assume a God exists to...well, disprove Him, or whatever else conclusion you want. It will just go in circles.

The argument of evil fails if there is no clear defined understanding of evil.

I think this would be a damn good argument:

The argument is valid, but it is not sound. Valid meaning that the premises follow for a valid conclusion, but the premises themselves are not accurate at all.

For premise 1, prove dragons are dedicated to destroying anything unlucky, and same goes for premise 2.

So how is it that this argument is a good one?

Likewise, an atheist who thinks there isn't anything objectively bad about animal suffering can still use animal suffering as evidence in an argument against the existence of any God who the theist alleges to be morally perfect and therefore opposed to things like animal suffering.

God isn't morally perfect. God cannot be moral. Morals come from His nature of being, but He is not moral. This word cannot logically apply to God, it can only apply to us humans.

If God was moral, then He is subject to higher morals beyond Himself.

And again, if an atheist would use animal suffering (which they do not believe is objectively bad), to argue against the existence of God, the theist will say that the reason they believe animal suffering is objectively bad is because it goes against the divine nature of essence of God Himself.

So the atheist would have to first accept that God exists...to prove that He doesn't exist. Now I don't know about you, but that seems like a fallacy. You cannot believe that something exists and not exists at the same time.